1. Field of the Invention
The field of the invention is that of illumination devices and more specifically that of trichromatic illumination devices suitable for illuminating an LCD screen.
2. Discussion of the Background
At the present time, many display applications require the use of compact and lightweight illumination devices, allowing both small screens and very large screens (having an area of greater than 1 square meter) to be illuminated.
For some applications, a major requirement is to optimize the compactness, light efficiency and contrast parameters, especially in applications such as ultralight imagers for avionic helmet displays or else for large wall-mounted flat television screens typically 5 cm in depth for an image of 1 meter in diagonal, operating in direct-view mode.
The current solutions for helmet imagers are in fact based on the use of monochromatic CRT mini tubes; however, an essential requirement in these applications is to display trichromatic images, as long as it is possible simultaneously to satisfy the compactness-weight and brightness criteria. With the present techniques, the use of an LCD screen has a certain advantage with regard to compactness-weight, nevertheless new lighting solutions have to be envisaged in order to provide sufficient luminance allowing the user to display an image with sufficient contrast (typically greater than 5), particularly when he is moving around in bright ambient light (a pilot on a daytime mission).
Likewise, in large television screen applications, the LCD solutions currently employed are based on projection techniques which result in not insignificant depths, ranging from 20 to 40 cm, in the case of image formats of about 1 meter in diagonal. By way of example, FIG. 1 illustrates an example of a display device using an image generator GI comprising especially a source and an active matrix, a projection optic OP and a deflecting mirror Mp which deflects the image towards the screen capital E, the whole system constituting a bulky device. One alternative would consist in using LCD screens in direct-view mode, based on a specific technology compatible with video-rate addressing, it being more difficult to address large pixels because of the large capacitances inherent in large electrooptic pixels. Large screens have the advantage of being able to dispense with a projection optic, but at the present time backlighting devices based on fluorescent tubes developed for computer applications do not have the luminance and contrast characteristics required for displaying a satisfactory video image in the mass market.